This invention pertains to an opto-mechanical scanning system for use in a two-dimensional imaging system, but more specifically, to a raster scanning reflector arrangement of the type including at least two reflectors that are driven to pivot upon axes orthogonal to one another for scanning elemental areas of a field.
In an imaging system which receives or projects radiant energy, an arrangement of reflectors and lenses is used for directing radiant energy from successive elemental areas of a scene to a suitable detector, or for projecting radiant energy from a source thereof to successive positions in a field. Some systems additionally use the reflector arrangement to project radiant energy to a target to illuminate it and thereby enhance the received image of the object.
Typical image enhancing radiation sources that might be used in imaging systems of the above type include coherent laser sources, non-coherent visable light sources, and thermal radiation sources (e.g., ultraviolet through infrared range). Detectors that might be used obviously should possess optical response characteristics that coincide with the wavelengths of the radiant energy being detected. Some of these detectors comprise a vertically aligned linear array of optically sensitive elements that detect radiation levels at successive vertical positions of a real image focused thereon by horizontally sweeping a corresponding object scene. Such a detector might be used in a scanning mirror arrangement having a single horizontal sweep mirror, instead of both a horizontal and a vertical sweep mirror. The vertical field of view in that type of scanning system is limited to the number of optically sensitive elements in the vertical detector array. Another system uses a single focal point detector wherein separator horizontal and vertical sweep reflectors are movable to pass successive elemental portions of a real image over the detector element. These reflectors are electromechanically actuated so that the real image, focused in a plane in which the detector is located, is moved in a raster fashion over the detector element. In this system, the vertical, as well as the horizontal, field of view is limited by the maximum angular displacement of the reflectors.
In a movable reflector mechanism using a single focal point detector element to sweep across the real image, a single gimballed reflector might ideally be used to attain the desired field of view in both vertical and horizontal directions. However, at high scanning frequencies, mechanical difficulties are experienced in the reflector actuating mechanism due to the difficulty in oscillating the relatively large mass of the inner gimbal assembly at correspondingly high frequencies. Therefore, two reflectors are generally used, each one being separately driven to oscillate about respective orthogonal axes.
Focusing of an object scene at the detector may be accomplished by placing an objective lens assembly in the optical path of the system, in which case, planar reflectors are usually used. Focusing can also be accomplished by providing curvature in one or both of the pivoting reflectors to thereby eliminate the need for the objective lens assembly. One disadvantage of using a focusing reflector results from focal plane errors, which increase with increased scan angle. Specifically, when a concave mirror is used as a focusing and scanning element, the points of elemental areas in the object scene mirror pivots about one axis. Thus, if a focal point detector is used to detect the radiation in successive points of a planar image, it will be positioned properly at the focal surface for only one angular position of the mirror. For all other angles the detector will not be on the focal surface, its distance therefrom increasing with the mirror angle. In contrast, when a planar mirror is used to scan the object scene, the focal points lie in a plane. A detector positioned in that plane will therefore retain its proper position relative to the image as the angular position of the lens changes.
Television compatibility is desired for some scanning mechanisms. Compatibility, without the use of expensive scan converters, requires that scanning pattern of the opto-mechanical scanner match the raster scanning pattern of the display circuits of a television video monitor. In a typical television system, horizontal line scanning of the brightness modulated electron beam is usually driven at a relatively high frequency sawtooth waveform, typically at a rate of about 15,750 cycles per second. Vertical scanning of the modulated electron beam is usually performed by a sawtooth waveform at a much lower frequency, typically at a rate near 30 cycles per second. Thus, to be compatible, the vertical and horizontal reflectors of the mechanical scanner must coincide, both in frequency and scanning pattern, with the vertical and horizontal scanning cycle of the television system. An example of a television compatible scanning system is described in U.S. Pat. No. 3,978,281 assigned to the assignee hereof. To achieve high mechanical scanning speeds, a television compatible opto-mechanical scanning system should use reflectors of relatively low mass. The high scanning rates of some high speed scanners impose stringent structural (e.g. stress, strain, and fatigue tolerance) and material (e.g. mass, size, and strength) requirements on the reflector elements and their associated drive mechanisms.
Image distortion presents yet another difficulty experienced in opto-mechanical scanning mechanisms that are interfaced with display monitors. Distortion results from differences between the scan pattern of the reflectors and the scan pattern of the electron beam in the display monitor. A conventional television monitor provides raster scanning in an x-y plane wherein x and y are perpendicular. To eliminate distortion, the scanning pattern of the reflectors in the opto-mechanical scanner should also be perpendicular. Such mechanical scanning would require that the axes of oscillation of the two reflectors be orthogonal. The optical throughput, that is, the quantity of radiant energy transferred through the reflector arrangement, should be as high as possible to lessen sensitivity requirements of the radiant energy detector, or in the case of a projector, to lessen the attenuation of the radiant energy source. Optical throughput can be increased by using larger reflectors and lenses, or by reducing obscuration in the optical path.
From the foregoing, it is quite apparent that several trade-offs among angle of scan (field of view), sensitivity, scan or frame rate, distortion, optical throughput, reflector size, and lens assemblies are considered in the design and construction of two-dimensional raster scanning reflector mechanisms. Other design tradeoffs concommitant with prior art scanners will become apparent upon review of this invention.
U.S. Pat. No. 3,704,342, issued to Stoddard et al on Nov. 28, 1972, describes a high speed infrared scanning mechanism incorporating a two-dimensional raster scanning mirror arrangement. The mechanism includes an objective lens assembly which focuses elemental areas of an object scene upon an infrared detector element. A planar vertical-scan mirror pivots upon a first horizontal axis and a planar horizontal-scan mirror pivots upon a second vertical axis that is orthogonal to the first axis. Resonant torsional oscillators drive the vertical-scan mirror approximately 5% from its nominal position to achieve a 10% vertical sweep at 30 Hz to produce pictorial frames, and drive the horizontal-scan mirror approximately 5% from its nominal position to achieve a 10% horizontal sweep at 3000 Hz to produce line scans within each frame.
In the arrangement of Stoddard et al, a light bundle from each successive elemental area of the object field being scanned first strikes the vertical-scan mirror, secondly strikes the horizontal-scan mirror, and is then focused on the detector by the objective lens assembly. In another embodiment, the light bundle is directed to a detector by a folding mirror.
Another scanning system is disclosed in U.S. Pat. No. 3,997,721 issued to Streifer on Dec. 14, 1976. It describes a method to reduce effective scan angle in a scanner for projecting a beam of light. The particular advantage accorded by Streifer is the reduction of focal plane errors when a proximal planar objective surface is scanned. While the angle of scan for proximal fields is increased, the actual field of view, or scan angle, for distal fields is decreased. The split spectrum field scanner of Streifer requires a plurality of additional reflectors, thus increasing the overall cost of construction, cost of maintenance, and accuracy of the system.
U.S. Pat. No. 3,816,741 issued to Macall describes yet another infrared scanning mechanism comprising a focusing concave vertical-scan mirror and a planar horizontal-scan mirror that pivot upon axes orthogonal to each other. Multiple reflections of radiant energy occur on the planar mirror which include a smaller obscuring reflector in the path of the radiant energy bundle. Multiple reflections increase the effective horizontal sweep angle in the object scene for a given angular sweep of the horizontal mirror. Thus a smaller scan angle for the mirror, which permits higher scan speeds for a given angle of scan, can be used. The smaller obscuring reflector however reduces the optical throughput of the system and thus requires either a more sensitive detector or larger reflectors. Larger reflectors may not be television compatible, for reasons previously indicated. Further, the placement of the smaller obscuring reflector between the vertical-scan and horizontal-scan mirrors prevents the scanning mirrors from being placed in close proximity to each other, and therefore reduces angle of scan attainable for given mirror sizes.
Additional drawbacks and disadvantages of prior art systems will become apparent upon review of the succeeding disclosure.